I. Field of the Invention
The present invention relates generally to tail wheels for small aircraft. More particularly, my invention relates to a tail wheel conversion system that eliminates jarring or jerking, and other unwanted vibration, thereby preventing tail wheel shimmying.
II. Description of the Prior Art
in general, the concept of stability relates to the characteristics of an aircraft in maintaining its course and direction. In flight, the term xe2x80x9cstabilityxe2x80x9d is often equated with the ability of the plane to fly itself. Stability can either be static or dynamic. Static stability involves only the return of the disturbed object to its original position. This was the goal of the early airplane designers; that the airplane would try to return to its original orientation (position) after a disturbance, such as a gust of wind. Dynamic stability is concerned with how much time it may take for the object to return to its original position. If the plane eventually returns to its original position, then the system is considered dynamically stable. If it does not, then it is considered dynamically unstable.
The concept of xe2x80x9ccontrolxe2x80x9d is a science relating to the human experience in flying and handling a given aircraft. The concepts are related, because when xe2x80x9ccontrolxe2x80x9d is optimized, a given airplane will be relatively easy for the pilot to fly, and highly stable in flight. The small airplanes used at local airports are very stable; they are (good for both beginning pilots and the more experienced pilots. They are very easy to fly and very forgiving of pilot mistakes. Although usually discussed as flight characteristic, stability and control are equally important during takeoff and landing.
When the plane is in contact with the runway, sudden movements to the left or right of the landing surface are disfavored. Stability dictates that the plane move forwardly and decelerate smoothly during a landing without sudden xe2x80x9cjerkiness.xe2x80x9d Similarly, as a plane taking off leaves the runway and breaks contact with the ground, irregular movements caused by runway-contact can affect flight path stability. One significant cause of vibration and jerkiness during takeoffs and landings is the tail wheel assembly, that can vibrate deleteriously when I contact with the runway.
While on the ground, static stability is enhanced by the normal three-point wheels of the aircraft. Usually a single tail wheel assembly at the aircraft rear completes the xe2x80x9cthird pointxe2x80x9d necessary for establishing a stable, planar position. With older small planes having xe2x80x9cfixedxe2x80x9d tail skid assemblies, the tail skid may be permanently oriented in a position parallel with the longitudinal axis of the airframe. In some tail wheel mounting designs, upward or downward movements of the tail wheel are enabled. However, if the tail wheels are mounted too loosely, they will shimmy or vibrate during takeoffs and landings. On the other hand, if they are secured too rigidly, and cannot xe2x80x9cgivexe2x80x9d in response to runway contact, proper handling can be negatively affected during landings. The time periods just before liftoff, and during landings, are often critical, and yet conventional tail wheel mounting assemblies allow the rear to vibrate.
The latter problem can be particular vexatious when operating agri-chemical dispensing airplanes from dirt runways. When the plane takes off, it is full of fuel and agri-chemical, at maximum weight. Jolts or jerking motions imparted by vibrating tail wheel assemblies during takeoffs can be disconcerting, to say the least. When the plane returns for a landing, it is much lighter, and flight characteristics are different than they were immediately after takeoff. During a landing, when the tail wheel makes first contact with the landing surface, a xe2x80x9csmoothxe2x80x9d and non-jerky transition is desirable. In the fraction of a second that the plane is neither fully airborne nor fully landed, the path of least resistance for wheel movement may not be straight down the runway. In other words, because of wind gusts and numerous other variables, a sleight movement in tail wheel orientation from xe2x80x9ctrue straightnessxe2x80x9d can decrease stability.
In other words, during takeoffs and landings especially, conventional tail wheel assemblies vibrate, rattle, and shimmy. If the tail wheel assembly is modified to prevent vibration, it must nevertheless be able to xe2x80x9cgivexe2x80x9d slightly when contacting the ground.
My new shimmy-preventing tail wheel conversion system is adapted for installation upon existing tail wheel or tail fork assemblies. Once installed, the tail wheel is free to rotate about the axis of the spindle in response to predetermined forces during takeoff and landing, but it cannot shimmy or vibrate during flight.
The conversion system must be operatively connected to the hollow, spindle shaft of the conventional tail wheel fork assembly. A rigid insert is pressed down into the upper end of the factory fork assembly. The insert is generally in the form of an inverted T, with an integral coaxially-centered stem projecting integrally upwardly from a cylindrical base. An elongated, shaft rigidly penetrates the insert and extends upwardly for attachment to a bearing assembly as described later. The insert""s cylindrical base is coaxially centered within, and welded to the tail wheel fork tube.
A hollow, sleeve-like hub is then coaxially coupled to the end of the fork assembly. The hub externally surrounds the insert. A lower, internal shoulder of the hub axially contacts and rests against the end of the fork tube. The insert shaft (preferably a threaded bolt) coaxially penetrates the hub and emanates upwardly therefrom. An upper hub shoulder receives a bearing assembly. The race portion is press-fitted within the shoulder, and the bearing is coaxially pressed against the race for rotation. The coaxially upwardly projecting threaded bolt receives a castellated nut that compresses the bearing assembly with suitable washers to maintain the desired degree of firmness.
While the tail wheel fork axle can rotate around its axis, substantial force is required to radially displace it. During landings or takeoffs, adequate forces are developed by runway contact so that the tail wheel orients itself normally. During flight, however, forces from wind, for example, are inadequate to rotate the tail wheel, and shimmy and vibration is avoided. Preferably the nut is tightened to approximately one hundred foot pounds of torque to preload the assembly. This xe2x80x9cpre-loadxe2x80x9d prevents the tail wheel from swinging freely and shimmying or vibrating. Uncontrolled shimmying results in dangerous vibration that can cause damage if continued unabated.
This synergistic xe2x80x9cpre-loadingxe2x80x9d of the stressed bearing actually acts as a xe2x80x9cbrakexe2x80x9d that makes it impossible for the tail wheel to shimmy and shake uncontrollably during the critical takeoff and landing rolls. The special bushing assures that the torque remains constant during normal operations.
Thus a basic object is to provide a shimmy-free tail wheel conversion system that can be easily retrofitted to existing tail wheel assemblies.
Another basic object is to provide an after-market system for stabilizing aircraft tail wheels to minimize shimmying and vibration.
Another important object is to provide a tail wheel mounting conversion of the character described that is suitable for user-installation.
Conversely, an important object is to provide a system of the character described that can be installed with new aircraft.
Yet another object is to provide a bushing construction for a system of the character described that both radially centers and axially stresses the bearings.
Another basic object is to provide a highly stable and impact-resistant tail wheel conversion for prior art aircraft tail wheels and forks.
Another important object is to provide an aircraft tail wheel assembly of the character described that makes it easier to take off and land, especially on irregular dirt runways or grass landing strips.
A related object is to minimize jarring or jerking effects.
A still further object is to minimize noise, and maximize strength.
These and other objects and advantages of the present invention, along with features of novelty appurtenant thereto, will appear or become apparent in the course of the following descriptive sections.